I was at ChefConf, the comment was made during a panel discussion on open source [1]. On the panel there was an engineer from Facebook, an IT Director from Gap, and Mark Russinovich the CTO of Azure (note Azure, not Windows).
The conversation went something like this (paraphrasing):
Moderator: "Microsoft used to really suck, and they were really anti-open source. But now they are open sourcing things like CLR on GitHub. I bet one thing they will never open source is Windows."
Mark: "You never know, it's definitely possible. Crazy stuff happens."
Nothing more on the subject.
[1] https://chefconf2015a.sched.org/event/00710f4fed0e8b617aef56...